For today’s post, I wrote about a paper published by the Colors of the Outer
Solar System Origins Survey team. They compiled a collection of 229 Kuiper Belt
Objects (KBOs) that all have well-measured colors. By “color,” I mean that the
astronomers obtained images of the KBOs in three different wavelength filters,
and since the objects might not reflect the same amount of light in each filter,
the difference between the three measurements gives an idea of the “color.” Just
like how plants appear green because most of the light they reflect is green
light.

There’s an additional complication, since the reflected light in question is the
Sun’s light. The Sun inherently has its own color, and so colors of KBOs tend to
be measured relative to the Sun’s color. Colors close to the Sun’s are referred
to as “solar neutral” or “gray,” while redder KBOs are referred to as “red.”
(There aren’t really any bluer-than-solar objects, though people sometimes
confusingly refer to gray KBOs as blue instead, mostly to mean “not-red.” But
the objects aren’t actually blue! Luckily the authors of this paper didn’t use
that terminology).

What’s so special about color? It turns out that an object’s color corresponds
to its composition, and its composition relates back to where in the early
planetary disk the object formed. Furthermore, other properties about the
object, such as its orbital inclination (how far the object swings out of the
plane of the eight major planets), also tell us about the object’s history. Did
it form closer to the Sun and get scattered out? Did it form farther out?
Considering both inclination and color together could be a powerful constraint
on where KBOs formed and how they got to where they are now!